1. Field of The Invention
The present invention related to a fixing device which is used for electrophotographic image foxing apparatuses to fix an ink material such as toner on a recording medium on which the material has been applied, and particularly to a fixing device for fixing an ink material on a recording medium by heating and pressing.
2. Description of The Prior Art
In most image forming apparatuses based on electrophotography, including copying machines, laser printers, etc., an ink material such as toner is applied to a recording medium directly or by means of a photoconductor, and such apparatuses are equipped with a fixing device for processing the ink material to fix it on the recording medium. A typical conventional fixing device is one which utilizes heat pressure fixation, that is, fixation by heat and pressure.
This heat pressure fixing device is equipped with a heat roller which generates heat and a press roller which contacts with the heat roller under a given pressure. FIG. 1 is a cross sectional view of a representative conventional heat pressure fixing device. In the drawing, the heat roller 301A is constructed from a heat-resistant, thin-walled, cylindrical drum, and an arc-like support guide 312 is placed inside the heat roller 301A along the area of contact between the heat roller 301A and press roller 302. A linear heating element 311 partially lines the inner surface of the arc-like support guide 312 while supported thereon. A paper guide 6 is arranged extending along the feed path for the sheet 2 as a recording medium, and a pair of discharge rollers 305 and 306 are placed downstream from the heat roller 301A and press roller 302 in such a manner that the pair of discharge rollers are pressed against each other. The heat roller 301A is equipped with a cleaning pad 308, a temperature sensor 310 and a release claw 309. The bearing section 303 of the press roller 302 is provided with a press spring 304 which functions as a pressure source.
When the heat roller 301A and press roller 302 are rotated in the direction indicted by the shown arrow, the sheet 2 with ink material to be fixed applied on its surface facing the heat roller 301A moves along the paper guide 6 and passes between the two rollers 301A and 302. Here, the ink material is fixed by being heated by the linear heating element 311 and pressed against the surface of the sheet. The sheet with the fixed ink material is then conveyed by the discharge rollers 305 and 306 and discharged through an outlet (not shown). In addition, the temperature sensor 310 detects the temperature of the heat roller 301A, and controls the amount of current to be fed to the linear heating element 311 as the heating source thereby controlling the heat roller 301A to a given temperature. Further, the cleaning pad 308 is provided to clean the surface of the heat roller 301A, and the release claw 309 is placed to prevent the sheet from wrapping the heat roller 301A. Heating of only portions of the heat roller 301A which come in contact with the linear heating element 311 provides the advantages of increased thermal efficiency and shortened warm-up time.
With this type of fixing device, however, since it is necessary to heat the drum-like heat roller 301A at a low temperature to a given temperature in a short time by the heat generated by the linear heating element 311, the linear heating element 311 must be a high-calorie type which becomes a bar against lowering in power consumption.
Another possible problem is lower heating efficiencies due to direct outward radiation of part of the heat generated by the linear heating element 311 which is expended without heating the heat roller 301A.
In addition, it is required to control the linear heating element 311 to a uniform temperature along its lengthwise direction because of direct influence of variations in temperature of the linear heating element 311 upon the fixation. Thus, there is presented the additional problem of difficult preparation and control of the linear heating element 311 due to required uniformity during the manufacturing process and temperature control with high accuracy during the operation.
Therefore, recently attention has been centered on heat pressure type fixing devices which do not use heating elements such as linear heating elements. This type of fixing device is designed to fix an ink material on a sheet by electromagnetic induction heating, and are disclosed in Japanese Examined Patent Application Publication HEI No. 5-9027 and Japanese Examined Patent Application Publication HEI No. 4-73155. The fixing device described in Japanese Examined Patent Application Publication HEI No. 5-9027 is equipped with a heat roller arranged in a closed magnetic circuit constructed from an excitation core and an auxiliary core, and is designed so that the heat roller is heated by Joule heat which is generated by eddy currents produced by a magnetic flux through the heat roller in its width direction. On the other hand, the fixing device described in Japanese Examined Patent Application Publication HEI No. 4-73155 comprises a closed magnetic circuit which is constructed from a looping magnetic member which surrounds the heat roller. The magnetic member is composed of a laminated core. An excitation coil is rolled around the magnetic member inside the heat roller so that the heat roller in the closed magnetic circuit undergoes induction heating by application of an AC signal to the excitation coil.
The aforementioned electromagnetic induction heating type of fixing device, however, requires placement of coiled laminated cores both inside and outside the heat roller, and this requires to upsize the diameter of the heat roller and eventually the fixing device since the cores themselves are large. An additional drawback is that the cores are heavy. Furthermore, with the fixing device, it is difficult to accomplish local heating of the heat roller; particularly it fails to heat the area of contact between the heat roller and press roller in a concentrated manner.